A variety of implantable medical devices (IMDS) provide electrical stimulation to cardiac muscle. Examples of such cardiac stimulators include pacemakers, defibrillators and cardiac resynchronization therapy (CRT) devices. Such devices may improve cardiac rhythm as well as cardiac function. For example, cardiac resynchronization therapy (CRT) has been clinically demonstrated to improve indices of cardiac function in patients suffering from congestive heart failure. CRT involves cardiac pacing that may be applied to one or both ventricles or multiple heart chambers, including one or both atria, to improve cardiac chamber coordination, which in turn is thought to improve pumping efficiency and stroke volume. Follow-up of patients undergoing CRT has shown improvements in clinical indices as well as hemodynamic measures of cardiac function, left ventricular volumes, and wall motion.
The benefits of CRT may derive from the synchronicity that it provides. For example, CRT allows left ventricular synchronicity such that the systolic phase, the diastolic phase and the mitral apparatus activation are coordinated. It also provides for atrio-ventricular synchronicity, resulting in improved diastolic filling and coordinated filling and ejection timing. However, in order to provide synchronicity and the associated benefits, the timing of the electrical stimulation by the CRT device must be appropriate. Therefore the selection of optimal pacing intervals is necessary to maximize these benefits.
Pacing intervals may be preset to a default setting at the time of implantation of the IMD device. Alternatively, selection of pacing intervals may be based on echocardiographic evaluation of cardiac function or a variety of other selection methods that attempt to optimize cardiac function or hemodynamic status. Some IMDs maintain fixed pacing intervals at all times. Others provide various intervals which are selected depending upon heart rate. While such pacing intervals may be adequate, they are not tailored to the individual patient or to the hemodynamic state of the patient at a particular time.
Because patients in need of cardiac stimulating IMDs are heterogeneous, the same pacing intervals may not be optimal for all patients. For example, some patients have ischemic heart disease while others do not. In addition, various cardiac factors may change throughout the day depending on the amount of activity of the patient and other factors. Thus the degree of ischemia, electrical activation, mechanical activation, loading conditions and degree of mitral insufficiency are different from patient to patient. In addition, these characteristics vary for individual patients throughout the day. Because of this variability, it is desirable for the pacing intervals to be optimized for each individual as well and to vary depending upon the hemodynamic state of that individual in order to optimize cardiac function.